Dual fuel question - curious

GavinC · Jul 9th, 2009, 17:46

960 with singlepoint Tartarini Tec '97...

On LPG... idled ultra smoothly and ran great, by and large. Through necessity in recent days it would start hot or cold on LPG, too... at least with a little cranking when cold. However would very occasionally cut out on LPG when coasting up to, say, an intersection. I'm aware of ram air effect and the airbox has been modified accordingly long since. Wondering if the stepper motor is 'sticky'... Or 'heavy ends' in vapouriser? (Ideas, anyone, please?) I've meanwhile noticed that the small ('idle bleed') screw on the vapouriser is closed - just, so not too damagingly tight. For now, I've left both vapouriser adjuster screws alone.

On PETROL, it WAS perfect - at least till just after MOT! After MOT... on petrol, would start but after first slightest touch on accelerator would idle very, very lumpily - cutting out unless coaxed (with great difficulty) with throttle. Once past 1200 rpm would run and pull fine, till drop in revs. (Consequently always having to start on LPG...)

So... checked (and misled!) by MAF... then cleaned Idle Air Control Valve. Appeared to be 'sticky.' Started up on petrol, no change to very lumpy, almost stalling idle. Then cleared OBD codes... Very next start - RESULT! Now idles smoothly and accelerates fine all the way through the rev range... on either fuel, too - as it did pre-MOT.

However no longer starts at all readily on LPG... anyway, doing so is not 'good LPG practice' for risk of lean backfire.

Just CURIOUS as to why cleaning the IACV appears to have dramatically improved idling and running behaviour below 1200rpm on petrol - as hoped for... while LPG - with the exception of starting - appears unaffected... 'sticky' IACV, or not.

Or is it simply happy coincidence... such as weak wiring I might have disturbed with my general playing around under the bonnet? I'm not sure if I yet fully understand the differences in the technical logic between the two fuel systems... in case of future challenges.

ANYONE, please?

richjc4 · Jul 9th, 2009, 19:13

whens the last time you changed the plugs and leads etc. The ignition needs to be tip top for lpg to be happy. Make sure you fit single electrode plugs too. you need to get the car running well on petrol first othwise the gas will stand no chance.

GavinC · Jul 9th, 2009, 21:52

Richjc4... Thanks - I was perhaps much too long winded in my post so you may have missed the gist of what I was trying to explain.

Single electrode (always) plugs are less than 5,000 miles and leads are all thoroughly checked. And connectors to the coils... And a failing coil pack - all relating to an ignition problem about 5 thou miles ago. (As you say, LPG is much more demanding on ignition than petrol.) Idles smooth as silk on LPG... could almost balance a coin on a level part of the engine without it falling over. Pulls like a train on LPG, too.

Just wondering whether, despite the attention to the IACV, it'd still 'cut out' without warning on LPG coasting up to, say, an intersection - albeit just once every couple hundred miles or so... Run it 99.9% time on LPG so cannot know if it has tendency to do same on petrol.

***My guess is I need to open the vapouriser 'idle bleed' screw a tiny tad to compensate for more air through IACV***

*** but I'd like somebody to corroborate that logic, please - I don't enjoy the cutting out!***

Despite being pleased to run this dual fuel 960 since 2002, I am sometimes still learning...

classicswede · Jul 9th, 2009, 22:11

The idle control valve will be in a different position while running on LPG. The area that is used for petrol will have gummed up and therfore not move as it should. Cleaning it has now allowed it to move freely and work fine while on petrol or LPG.

Hope that explains it to you.

Clifford Pope · Jul 10th, 2009, 17:35

Also have you cleaned the throttle body recently? At idling speed there is only a tiny clearance round the butterfly, so small changes in the amount of muck there will have big effects. At MOTs they often rev the engine vigorously, so may have dislodged something.
Idling on gas feeds a gas/air mixture round the IAV. Idling on petrol feeds only air, petrol being injected further downstream.
(he says knowledgably, having only just learned that!)

Jul 9th, 2009, 17:46	#1
GavinC GavinC Last Online: Oct 13th, 2012 18:51 Join Date: Dec 2003 Location: Harrogate	Dual fuel question - curious 960 with singlepoint Tartarini Tec '97... On LPG... idled ultra smoothly and ran great, by and large. Through necessity in recent days it would start hot or cold on LPG, too... at least with a little cranking when cold. However would very occasionally cut out on LPG when coasting up to, say, an intersection. I'm aware of ram air effect and the airbox has been modified accordingly long since. Wondering if the stepper motor is 'sticky'... Or 'heavy ends' in vapouriser? (Ideas, anyone, please?) I've meanwhile noticed that the small ('idle bleed') screw on the vapouriser is closed - just, so not too damagingly tight. For now, I've left both vapouriser adjuster screws alone. On PETROL, it WAS perfect - at least till just after MOT! After MOT... on petrol, would start but after first slightest touch on accelerator would idle very, very lumpily - cutting out unless coaxed (with great difficulty) with throttle. Once past 1200 rpm would run and pull fine, till drop in revs. (Consequently always having to start on LPG...) So... checked (and misled!) by MAF... then cleaned Idle Air Control Valve. Appeared to be 'sticky.' Started up on petrol, no change to very lumpy, almost stalling idle. Then cleared OBD codes... Very next start - RESULT! Now idles smoothly and accelerates fine all the way through the rev range... on either fuel, too - as it did pre-MOT. However no longer starts at all readily on LPG... anyway, doing so is not 'good LPG practice' for risk of lean backfire. Just CURIOUS as to why cleaning the IACV appears to have dramatically improved idling and running behaviour below 1200rpm on petrol - as hoped for... while LPG - with the exception of starting - appears unaffected... 'sticky' IACV, or not. Or is it simply happy coincidence... such as weak wiring I might have disturbed with my general playing around under the bonnet? I'm not sure if I yet fully understand the differences in the technical logic between the two fuel systems... in case of future challenges. ANYONE, please?

Jul 9th, 2009, 22:11	#4
classicswede Trader Volvo in my veins Last Online: Yesterday 23:53 Join Date: Dec 2004 Location: Anglesey	The idle control valve will be in a different position while running on LPG. The area that is used for petrol will have gummed up and therfore not move as it should. Cleaning it has now allowed it to move freely and work fine while on petrol or LPG. Hope that explains it to you. __________________ http://www.classicswede.org www.classicswede.co.uk

Jul 9th, 2009, 19:13	#2
richjc4 Junior Member Last Online: Apr 21st, 2012 20:07 Join Date: Jul 2009 Location: bham	whens the last time you changed the plugs and leads etc. The ignition needs to be tip top for lpg to be happy. Make sure you fit single electrode plugs too. you need to get the car running well on petrol first othwise the gas will stand no chance.

Jul 9th, 2009, 21:52	#3
GavinC GavinC Last Online: Oct 13th, 2012 18:51 Join Date: Dec 2003 Location: Harrogate	Richjc4... Thanks - I was perhaps much too long winded in my post so you may have missed the gist of what I was trying to explain. Single electrode (always) plugs are less than 5,000 miles and leads are all thoroughly checked. And connectors to the coils... And a failing coil pack - all relating to an ignition problem about 5 thou miles ago. (As you say, LPG is much more demanding on ignition than petrol.) Idles smooth as silk on LPG... could almost balance a coin on a level part of the engine without it falling over. Pulls like a train on LPG, too. Just wondering whether, despite the attention to the IACV, it'd still 'cut out' without warning on LPG coasting up to, say, an intersection - albeit just once every couple hundred miles or so... Run it 99.9% time on LPG so cannot know if it has tendency to do same on petrol. *My guess is I need to open the vapouriser 'idle bleed' screw a tiny tad to compensate for more air through IACV* * but I'd like somebody to corroborate that logic, please - I don't enjoy the cutting out!* Despite being pleased to run this dual fuel 960 since 2002, I am sometimes still learning...

Jul 10th, 2009, 17:35	#5
Clifford Pope Not an expert but ... Last Online: Yesterday 12:45 Join Date: Dec 2001 Location: Boncath	Also have you cleaned the throttle body recently? At idling speed there is only a tiny clearance round the butterfly, so small changes in the amount of muck there will have big effects. At MOTs they often rev the engine vigorously, so may have dislodged something. Idling on gas feeds a gas/air mixture round the IAV. Idling on petrol feeds only air, petrol being injected further downstream. (he says knowledgably, having only just learned that!)

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